Electrostatic-capacity-type acceleration sensor and acceleration measuring device therewith

ABSTRACT

A moving member having a plurality of moving electrodes is supported by support members at both ends thereof on a substrate surface in such a way that it can be subjected to displacement in a two-dimensional plane. A plurality of fixed electrodes are arranged to face the plurality of moving electrodes respectively, thus forming different facing areas therebetween when an input acceleration is zero. The facing areas formed between pairs of the electrodes facing each other are varied in response to the displacement of the moving member, whereby a capacitance caused by one pair of the electrodes whose facing area is relatively small is used to detect a relatively small input acceleration, and a capacitance caused by the other pair of the electrodes whose facing area is relatively large is used to detect a relatively large input acceleration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrostatic-capacity-type acceleration sensors that detect accelerations based on variations in electrostatic capacity (or capacitance), and it also relates to acceleration measuring devices using electrostatic-capacity-type acceleration sensors.

This application claims priority on Japanese Patent Application No. 2003-55562, the content of which is incorporated herein by reference.

2. Description of the Related Art

Conventionally, a typical type of an electrostatic-capacity-type acceleration sensor (as disclosed in Japanese Patent Application Publication No. Hei 7-260510) comprises a capacitance detector having a moving electrode and a fixed electrode, which are arranged opposite to each other such that a separating distance therebetween is varied in response to an input acceleration, wherein a switched-capacitor circuit converts capacitance variation due to displacement of the moving electrode into voltage variation so as to detect and output an acceleration signal, and wherein in order to broaden the detection range, a servo circuit is provided so as to hold the moving electrode at a neutral position by feeding back the acceleration signal.

Another type of the electrostatic-capacity-type acceleration sensor (as disclosed in Japanese Patent Application Publication No. Hei 10-206457) comprises a moving electrode and a fixed electrode, which are arranged on a substrate surface such that a facing area therebetween is varied in response to an input acceleration, wherein based on the capacitance realized by the moving electrode and fixed electrode, the capacitance variation due to the displacement of the moving electrode is detected so as to produce an acceleration signal.

The acceleration sensor having the servo circuit makes it possible to broaden the detection range compared with the acceleration sensor not equipped with the servo circuit because the movement of the moving electrode is limited in response to the output of the servo circuit. However, the range of the acceleration realized by the servo circuit stabilizing the moving electrode is limited by the electrostatic attraction and the weight of the moving electrode; hence, it is not easy to broaden the detection range.

The acceleration sensor in which the facing area between the moving electrode and fixed electrode is varied provides only a single kind of variation characteristic with regard to the facing area responding to the input acceleration; hence, it is not easy to broaden the detection range.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a brand-new electrostatic-capacity-type acceleration sensor whose detection range can be broadened with ease.

An electrostatic-capacity-type acceleration sensor of this invention comprises a plurality of capacitance detectors having pairs of moving electrodes and fixed electrodes that are arranged to face each other on the substrate surface, wherein facing areas are varied in response to an input acceleration. Herein, the capacitance detectors are constituted such that the facing areas between the moving electrodes and fixed electrodes differ from each other when the input acceleration is zero, so that each of the capacitance detectors produces an acceleration signal in response to the capacitance variation occurring between the paired electrodes thereof.

Since the acceleration sensor employs the aforementioned constitution adapted in each of the capacitance detectors in which the facing areas between the moving electrodes and fixed electrodes differ from each other so as to produce a variety of acceleration signals in response to capacitance variations occurring between the paired electrodes, it is possible to easily broaden the overall detection range for detecting the input acceleration by simply increasing the number of the capacitance detectors installed therein, wherein it is possible to provide each capacitance detector with a specific detection range suited thereto, thus realizing a high accuracy in the detection of the acceleration.

In the above, all of the moving electrodes provided for the capacitance detectors can be integrally formed together. Thus, it is possible to improve the integration of components and parts in the manufacturing of the acceleration sensor on the substrate surface by using the modern semiconductor manufacturing technology.

Specifically, each of the capacitance detectors comprises two pairs of moving electrodes and fixed electrodes, which are arranged in such a way that the facing area between the first moving electrode and first fixed electrode increases while the facing area between the second moving electrode and second fixed electrode decreases in response to the same input acceleration, whereby it produces an acceleration signal in response to a ratio or a difference between the first capacitance actualized between the first moving electrode and first fixed electrode and the second capacitance actualized between the second moving electrode and second fixed electrode. Thus, it is possible to noticeably improve the detection sensitivity in each of the capacitance detectors because the acceleration signal can be precisely produced in response to the ratio or difference between the two types of the capacitance.

In addition, the capacitance detectors can be constituted such that one capacitance detector has a relatively small facing area between the paired electrodes so as to produce an acceleration signal in response to a relatively small input acceleration, and the other capacitance detector has a relatively large facing area between the paired electrodes so as to produce an acceleration signal in response to a relatively large input acceleration. Thus, it is possible to detect the input acceleration with a high sensitivity in a relatively broad detection range.

An acceleration measuring device of this invention comprises an electrostatic-capacity-type acceleration sensor including first and second capacitance detectors each having a pair of a moving electrode and a fixed electrode in which the facing area between the paired electrodes in the second capacitance detector is set to be larger than the facing area between the paired electrodes in the first capacitance detector when an input acceleration is zero, a first detection circuit for producing a first acceleration signal in response to capacitance variation occurring between the moving electrode and fixed electrode in the first capacitance detector, a second detection circuit for producing a second acceleration signal in response to capacitance variation occurring between the moving electrode and fixed electrode in the second capacitance detector, and a selector for selectively outputting the first acceleration signal as long as the first acceleration signal does not exceed a threshold level determined in advance and for selectively outputting the second acceleration signal when the first acceleration signal exceeds the threshold level.

The aforementioned acceleration measuring device can measure acceleration at a high accuracy in a relatively broad detection range. The threshold value set to the selector can be determined to exclude a sharp varying region in a capacitance varying characteristic established with respect to variations of the input acceleration; hence, it is possible to reliably avoid occurrence of error detection due to noise.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, aspects, and embodiments of the present invention will be described in more detail with reference to the following drawings, in which:

FIG. 1A shows a schematic arrangement of electrodes in a capacitance detector in which facing areas between moving electrodes and fixed electrodes are decreased in response to acceleration;

FIG. 1B shows a schematic arrangement of electrodes in a capacitance detector in which facing areas between moving electrodes and fixed electrodes are increased in response to acceleration;

FIG. 2 is a graph showing curves representing relationships between the input acceleration and capacitance varying ratio (C_(U)/C_(D)) with respect to various electrode overlap values (L);

FIG. 3 is a plan view showing a layout of parts constituting an electrostatic-capacity-type acceleration sensor in accordance with a preferred embodiment of the invention; and

FIG. 4 is a block diagram showing an example of an acceleration measuring circuit using the electrostatic-capacity-type acceleration sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention will be described in further detail by way of examples with reference to the accompanying drawings.

FIGS. 1A and 1B show examples of arrangements of electrodes in capacitance detectors used for an electrostatic-capacity-type acceleration sensor in accordance with the preferred embodiment of the invention, wherein FIG. 1A shows an arrangement of electrodes in the facing area decreasing side, and FIG. 1B shows an arrangement of electrodes in the facing area increasing side. With reference to FIGS. 1A and 1B, the operating principle of the electrostatic-capacity-type acceleration sensor will be described.

In FIGS. 1A and 1B, each of moving electrodes M₁ to M₄ has a rectangular shape in which the length is set to ‘2L’ and the width is set to a certain value. Herein, the moving electrodes M₁ and M₃ are arranged along a dashed line L₁ in parallel with a prescribed distance therebetween such that the centers in their length directions match the dashed line L₁. Similarly, the moving electrodes M₂ and M₄ are arranged along a dashed line L₂ in parallel with a prescribed distance therebetween such that the centers in their length directions match the dashed line L₂. The dashed lines L₁ and L₂ are drawn in parallel with each other. In FIG. 1A, the moving electrodes M₁ and M₂ are arranged in parallel with an electrode pitch ‘4L’ therebetween. In FIG. 1B, the moving electrodes M₃ and M₄ are arranged in parallel with an electrode pitch ‘4L’ therebetween. Each of the moving electrodes M₁ to M₄ has a deadweight function, so that they can be collectively subjected to displacement in left-right directions with respect to the drawing sheet of FIGS. 1A and 1B in response to an input acceleration while maintaining the aforementioned relationship of arrangement.

In FIG. 1A, fixed electrodes S₁ and S₂ are respectively arranged under the moving electrodes M₁ and M₂, wherein the fixed electrode S₁ is arranged to oppositely face the left-half portion (whose length is ‘L’) of the moving electrode M₁, and the fixed electrode S₂ is arranged to oppositely face the left-half portion (whose length is ‘L’) of the moving electrode M₂. For the sake of convenience, the fixed electrodes S₁ and S₂ are drawn not to overlap with the moving electrodes M₁ and M₂ in FIG. 1A. Similarly, in FIG. 1B, fixed electrodes S₃ and S₄ are respectively arranged under the moving electrodes M₃ and M₄, wherein the fixed electrode S₃ is arranged to oppositely face the right-half portion (whose length is ‘L’) of the moving electrode M₃, and the fixed electrode S₄ is arranged to oppositely face the right-half portion (whose length is ‘L’) of the moving electrode M₄. For the sake of convenience, the fixed electrodes S₃ and S₄ are drawn not to overlap with the moving electrodes M₃ and M₄ in FIG. 1B.

In the above, there are provided four pairs of the fixed electrodes and the moving electrodes oppositely facing each other, namely, M₁-S₁, M₂-S₂, M₃-S₃ , and M₄-S₄, each pair of which realizes a certain capacitance. At 0 G where the input acceleration is zero, each pair realizes the same capacitance ‘C₀’. Next, suppose that the input acceleration of 1 G is applied on the moving electrodes M₁ to M₄, all of which are moved rightwards as indicated by arrows in FIGS. 1A and 1B, and each of which is thus subjected to displacement in a distance ‘d’, the “paired” electrodes (i.e., M₁-S₁, and M₂-S₂) decrease the facing areas therebetween so that their capacitance decrease from C₀ to C_(D) in FIG. 1A, while the other “paired” electrodes (i.e., M₃-S₃, and M₄-S₄) increase the facing areas therebetween so that their capacitances increase from C₀ to C_(U). Therefore, when the input acceleration acting on the moving electrodes M₁ to M₄ becomes greater than 1 G the capacitance C_(D) further decreases, and the capacitance C_(U) further increases.

FIG. 2 is a graph showing relationships between the input acceleration and the capacitance varying ratio C_(U)/C_(D) with respect to various electrode overlap values, which are related to the electrode facing area ‘L’. That is, the graph of FIG. 2 provides eight curves K₁, K₂, K₃, K₄, K₅, K₆, K₇, and K₈, each of which shows the relationship between the input acceleration and the capacitance varying ratio C_(U)/C_(D) with regard to the electrode overlap value L, which is set to 0.5 μm, 0.75 μm, 1.0 μm, 1.25 μm, 1.5 μm, 2.0 μm, 2.5 μm, and 5.0 μm respectively.

According to the graph of FIG. 2, as the electrode overlap value L becomes greater, it becomes possible to detect ‘greater’ input acceleration. For this reason, the acceleration sensor of the present embodiment comprises a plurality of capacitance detectors having different electrode overlap values, each of which is adequately constituted to detect input acceleration in a certain detection range suited thereto. That is, a relatively small input acceleration can be detected using a capacitance detector whose electrode overlap value is relatively small, and a relatively large input acceleration is detected using a capacitance detector whose electrode overlap value is relatively large. Hence, it is possible to actualize a high sensitivity in detecting acceleration in a relatively broad range of detection.

With respect to each of the aforementioned curves (e.g., K1), in order to avoid error detection due to noise, it is preferable not to use a certain region in which the capacitance varying ratio C_(U)/C_(D) becomes large so as to make variation sharp.

FIG. 3 shows an electrostatic-capacity-type acceleration sensor 10 in accordance with the preferred embodiment of the invention.

In the acceleration sensor 10, a moving member MB having a deadweight function is arranged on the surface of a silicon substrate and is supported by four support members H₁ to H₄ at both sides thereof such that it can be subjected to displacement in a direction DS in parallel with the substrate surface. The ends of the support members H₁ to H₄ are fixed at prescribed positions by fixing members P₁ to P₄ respectively. Four moving electrodes M₁₁, M₂₁, M₂₄, and M₁₄ are arranged on one side of the moving member MB and are projected in parallel with the substrate surface. In addition, four moving electrodes M₁₃, M₂₃, M₂₂, and M₁₂ are arranged on the other side of the moving member MB and are projected in parallel with the substrate surface. All of the moving electrodes M₁₁, to M₁₄ have the same length projecting from the moving member MB, and all of the moving electrodes M₂₁ to M₂₄ have the same length projecting from the moving member MB. In addition, all of the moving electrodes M₁₁ to M₁₄ have the same width, and all of the moving electrodes M₂₁ to M₂₄ have the same width that is greater than the width of the moving electrodes M₁₁ to M₁₄.

All of the moving member MB, the moving electrodes M₁₁–M₁₄ and M₂₁–M₂₄, and the support members H₁–H₄ are integrally combined together as a single assembly. Such an assembly can be formed by patterning conductive layers, which are made of a semiconductor or a metal and are deposited on an insulating film covering the substrate surface, wherein after the patterning, the insulating film is removed to allow movements of the moving member MB, the moving electrodes M₁₁–M₁₄ and M₂₁–M₂₄, and the support members H₁–H₄, for example. In addition, four fixing holes are formed at four fixing positions on the insulating film covering the substrate surface and are filled with conductive plugs, made of a semiconductor or a metal, thus forming the four fixing members P₁ to P₄.

Four fixed electrodes S₁₁ to S₁₄ are arranged opposite to the moving electrodes M₁₁ to M₁₄ with relatively small facing areas therebetween, thus actualizing capacitances C₁₁ to C₁₄ having substantially the same value when the input acceleration is zero. In addition, four fixed electrodes S₂₁ to S₂₄ are arranged opposite to the moving electrodes M₂₁ to M₂₄ with relatively large facing areas therebetween, thus actualizing capacitances C₂₁ to C₂₄ having substantially the same value when the input acceleration is zero. Suppose that the moving member MB is subjected to displacement in the direction DS as indicated by an arrow in FIG. 4 (i.e., a forward direction in parallel with the sheet of FIG. 4), wherein the fixed electrodes S₁₁, S₁₂, S₂₁, and S₂₂ are associated with the moving electrodes M₁₁, M₁₂, M₂₁, and M₂₂ such that t C₁₁, C₁₂, C₂₁, and C₂₂ respectively, while the fixed electrodes S₁₃, S₁₄, S₂₃, and S₂₄ associated with the moving electrodes M₁₃, M₁₄, M₂₃, and M₂₄ such that the facing areas therebetween are increased so as to increase the capacitances C₁₃, C₁₄, C₂₃, and C₂₄ respectively. In FIG. 4, the capacitances C₁₁–C₁₄ and C₂₁–C₂₄ are associated with arrows, wherein a downward slanted arrow indicates that the corresponding capacitance is decreased, and an upward slanted arrow indicates that the corresponding capacitance is increased.

The aforementioned fixed electrodes S₁₁–S₁₄ and S₂₁–S₂₄ are formed by impurity-doped regions in which conductive impurities are selectively doped onto the substrate surface. Alternatively, they are formed by patterning conductive layers, made of a semiconductor or a metal, which are deposited on an insulating film covering the substrate surface.

Wiring layers W₁₁–W₁₄ and W₂₁–W₂₄ are respectively extended from the fixed electrodes S₁₁–S₁₄ and S₂₁–S₂₄. The wiring layers W₁₁ and W₁₂ are interconnected with a detection line Ta via connection members Q₁₁ and Q₁₂ respectively. The wiring layers W₁₃ and W₁₄ are interconnected with a detection line Tb via connection members Q₁₃ and Q₁₄ respectively. The wiring layers W₂₁ and W₂₂ are interconnected with a detection line Tc via connection members Q₂₁ and Q₂₂ respectively. The wiring layers W₂₃ and W₂₄ are interconnected with a detection line Td via connection members Q₂₃ and Q₂₄ respectively. The support member H₁ is connected with a detection line Te.

The wiring layers W₁₁–W₁₄ and W₂₁–W₂₄ are formed by patterning such that their widths are reduced to be as small as possible in order to reduce parasitic capacitances thereof; the distances with the moving parts (i.e., MB, M₁₁–M₁₄, M₂₁–M₂₄) are increased to be as large as possible; and the electrode overlap values associated with the moving parts are reduced to be as small as possible. Similar to the fixed electrodes S₁₁–S₁₄ and S₂₁–S₂₄, the wiring layers W₁₁–W₁₄ and W₂₁–W₂₄ are formed by impurity-doped regions or conductive layers subjected to patterning. Eight connection holes are formed at prescribed positions on the insulating film covering the substrate surface and are filled with conductive plug, made of a semiconductor or a metal, thus actualizing the connection members Q₁₁–Q₁₄ and Q₂₁–Q₂₄. Incidentally, the connection members (e.g., Q₁₁) can be formed by adopting the aforementioned treatment for use in the formation of the fixing members (e.g., P₁).

For example, after the wiring layers W₁₁–W₁₄ and W₂₁–W₂₄ are formed as impurity-doped regions, a first wiring layer forming process is performed on the insulating layer covering the substrate surface so as to form the fixed electrodes S₁₁–S₁₄ and S₂₁–S₂₄; then, a second wiring layer forming process is performed on the insulating film covering the fixed electrodes S₁₁–S₁₄ and S₂₁–S₂₄ so as to form the moving member MB, the moving electrodes M₁₁–M₁₄ and M₂₁–M₂₄, the support members H₁–H₄, and the detection lines Ta–Td. Thus, it is possible to increase distances between the wiring layers W₁₁–W₁₄ and W₂₁–W₂₄ and the moving parts (i.e., MB, M₁₁–M₁₄, M₂₁–M₂₄); hence, it becomes possible to reduce parasitice capacitances.

FIG. 4 shows an equivalent circuit of the acceleration sensor 10 shown in FIG. 3; specifically, it shows an example of an acceleration measuring circuit using the acceleration sensor 10 shown in FIG. 3. The acceleration measuring circuit of FIG. 4 is designed to measure the input acceleration in a range between 0 G and 8 G as shown in FIG. 2, wherein the curve K₇ is used for the measurement of a relatively small input acceleration that ranges from 0 G to 4 G, and the curve K₈ is used for the measurement of a relatively large input acceleration that ranges from 4 G to 8 G. Specifically, in a range R₁ wherein 0 G≦input acceleration≦4 G, compared with the curve K₈, the curve K₇ provides a greater value of the capacitance varying ratio C_(U)/C_(D), which in turn actualizes a high accuracy in measurement. In a range R₂ where 4 G<input acceleration≦8 G, the curve K₇ includes a measurement-incapable region and is not used in measurement, whereas the curve K₈ is suitable in measurement. In order to avoid error detection due to noise, it is necessary not to use a sharp varying region of the curve K₇ with respect to the range R₁, and it is necessary not to use a sharp varying region of the curve K₈ with respect to the great input acceleration over 8 G, wherein these regions are excluded from the measurement.

A detection circuit 12A is connected with the detection lines Ta, Tb, and Te; and a detection circuit 12B is connected with the detection lines Tc, Td, and Te. Suppose that the capacitance varying characteristic of the capacitance detector, including the capacitances C₁₁ to C₁₄, against the input acceleration is shown by the curve K₇ shown in FIG. 2, while the capacitance varying characteristic of the capacitance detector, including the capacitances C₂₁ to C₂₄, against the input acceleration is shown by the curve K₈ shown in FIG. 2, for example. Herein, the capacitance varying ratio C_(U)/C_(D) is calculated using C₁₁–C₁₄ and C₂₁–C₂₄ as follows: $\frac{C_{13} + C_{14}}{C_{11} + C_{12}}$ $\frac{C_{23} + C_{24}}{C_{21} + C_{22}}$

With reference to the capacitance varying characteristic of the curve K₇, the detection circuit 12A produces an acceleration signal AS₁, representing the voltage related to a ratio(or a difference) between (C₁₃+C₄₁) and (C₁₁+C₁₂). With reference to the capacitance varying characteristic of the curve K₈, the detection circuit 12B produces an acceleration signal AS₂ representing the voltage related to a ratio (or a difference) between (C₂₃+C₂₄) and (C₂₁+C₂₂). The acceleration signal AS₁ is increased in level as the input acceleration increases from 0 G The acceleration signal AS₂ is set such that it substantially matches the acceleration signal AS₁ in level at the input acceleration of 4 G, and it is further increased from that level as the input acceleration increases from 4 G.

A selector 14 has terminals A and B for receiving signals as well as a select terminal SB. Herein, the selector 14 selectively outputs the acceleration signal SA₁ when SB is low (or at ‘0’), while it selectively outputs a signal SO₂, which is output from a selector 18, when SB is high (or at ‘1’). A comparator 16 compares the acceleration signal AS₁ with reference voltage V_(R1) so as to produce a comparison output CO₁, which is supplied to the select terminal SB of the selector 14. That is, the comparator 16 outputs CO₁=0 when AS₁≦V_(R1); and it outputs CO₁=1 when AS₁>V_(R1).

The prescribed voltage related to the capacitance varying ratio C_(U)/C_(D), which is read from the curve K₇ in FIG. 2 when the input acceleration is 4 G, is given as the reference voltage V_(R1) supplied to the comparator 16. As a result, the selector 14 provides a selection output SO₁ corresponding to the acceleration signal AS₁, which is produced in response to the input acceleration in the range R₁ where 0 G ≦input acceleration≦4 G.

The selector 18 has terminals A and B for receiving signals as well as a select terminal SB. That is, the selector 18 selectively outputs the acceleration signal SA₂ when SB is low (or at ‘0’), while it selectively outputs an out-of-range signal ASx representing that the input acceleration is out of the measurement range when SB is high (or at ‘1’). A comparator 20 compares the acceleration signal AS₂ with a reference voltage V_(R2) so as to provide a comparison output CO₂ to the select terminal SB of the selector 18. That is, the comparator 20 outputs CO₂=0 when AS₂≦V_(R2); and it outputs CO₂=1 when AS₂>V_(R2).

The prescribed voltage related to the capacitance varying ratio C_(U)/C_(D), which is read from the curve K₈ in FIG. 2 when the input acceleration is 8 G is given as the reference voltage V_(R2) supplied to the comparator 20. As a result, the selector 18 provides a selection output SO₂ corresponding to the acceleration signal AS₂, which is produced in response to the input acceleration in the range R₂ where 4 G<input acceleration≦8 G. In this case, the selector 14 is set so as to select the terminal B for receiving the selection output SO₂ of the selector 18 in response to the comparison output CO₁ of the comparator 16; hence, the selector 14 provides the selection output SO₁ corresponding to the acceleration signal AS₂.

When the acceleration signal AS₂ exceeds the reference voltage V_(R2) (where AS₂>V_(R2)), the comparison output CO₂ of the comparator 20 turns to ‘1’; hence, the selector 18 provides the selection output SO₂ corresponding to the out-of-range signal ASx. In this case, the selector 14 is set so as to select the terminal B for receiving the selection output SO₂ of the selector 18; hence, it provides the selection output SO₁ corresponding to the out-of-range signal Asx. An acceleration display (not shown) is provided to display an acceleration based on the acceleration signal AS₁ or AS₂ and to also display a message that the input acceleration is out of the measurement range in response to the out-of-range signal ASx.

The aforementioned acceleration measuring circuit of FIG. 4 is designed to perform measurement at a high accuracy with respect to a relatively broad range of 0 G ≦input acceleration≦8 G, which is realized by the combination of the aforementioned ranges R₁ and R₂. Herein, the measurement regarding the input acceleration is performed by excluding sharp varying regions of the curves K₇ and K₈ shown in FIG. 2; hence, it is possible to reliably avoid occurrence of error detection due to noise. The acceleration measuring circuit of FIG. 4 is designed selectively using two curves K₇ and K₈ within the eight curves K₁ to K₈ shown in FIG. 2. Of course, it is possible to easily modify the acceleration measuring circuit using three or more curves.

As described heretofore, an acceleration sensor of this invention is designed such that a plurality of capacitance detectors including pairs of moving electrodes and fixed electrodes in which facing areas therebetween differ from each other when the input acceleration is zero are arranged on the substrate surface, wherein each of the capacitance detectors is set to a prescribed measurement range of acceleration. Hence, by simply increasing the number of capacitance detectors, it is possible to actualize broadening of the overall detection range of acceleration for an acceleration measuring circuit; thus, it is possible to realize acceleration detection at a high sensitivity and in a relatively broad range.

As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims. 

1. An acceleration measuring device comprising: an electrostatic-capacity-type acceleration sensor including a first capacitance detector having plural pairs of moving electrodes and fixed electrodes, which are arranged to face each other on a substrate surface such that facing areas therebetween are varied in response to an input acceleration, and a second capacitance detector having plural pairs of moving electrodes and fixed electrodes, which are arranged to face each other on the substrate surface such that facing areas therebetween are varied in response to the input acceleration and are set to be larger than the facing areas between the moving electrodes and the fixed electrodes of the first capacitance detector when the input acceleration is zero; a first detection circuit for producing a first acceleration signal in response to capacitance variation occurring between the moving electrodes and the fixed electrodes of the first capacitance detector; a second detection circuit for producing a second acceleration signal in response to capacitance variation occurring between the moving electrodes and the fixed electrodes of the second capacitance detector; and a selector for selectively outputting the first acceleration signal as long as the first acceleration signal does not exceed a threshold level determined in advance and for selectively outputting the second acceleration signal when the first acceleration signal exceeds the threshold level.
 2. The acceleration measuring device according to claim 1, wherein a relatively small input acceleration is detected based on the first acceleration signal produced by the first capacitance detector, while a relatively large input acceleration is detected based on the second acceleration signal produced by the second capacitance detector.
 3. The acceleration measuring device according to claim 1, wherein each of the first and second capacitance detectors comprises first and second moving electrodes, and first and second fixed electrodes, which are respectively arranged to face each other with first and second facing areas therebetween in such a way that the first facing area formed between the first moving electrode and the first fixed electrode decreases while the second facing area formed between the second moving electrode and the second fixed electrode increases in response to the input acceleration, whereby each of the capacitance detectors produces an acceleration signal in response to a ratio or a difference between a first capacitance occurring between the first moving electrode and the first fixed electrode and a second capacitance occurring between the second moving electrode and the second fixed electrode.
 4. The acceleration measuring device according to claim 1, wherein the threshold level is determined in advance to exclude a sharp varying region of a capacitance varying characteristic established with respect to variations of the input acceleration, thus avoiding occurrence of error detection due to noise. 